As a technique for low power consumption, there is known a technique for reducing a power supply voltage during a standby or low-speed operation of a memory system. However, the technique has a problem that a plurality of external power supplies are needed for supplying the power supply voltage. Further, a switch is used for changing over the power supply voltage, and when switch control is not sufficiently taken into consideration, a temporary interruption of the power supply or a rapid variation in voltage in a transition state can occur. For the memory system, destroyed memory data caused by the temporary interruption of the power supply or the rapid variation in voltage is a serious problem.